This invention relates to a Fresnel lens sheet for use with a transmission type screen and the like, a transmission type screen for use with a rear projection type display apparatus and the like, and a rear projection type display apparatus.
A rear projection type display apparatus has spread widely as a kind of image display apparatus having a large screen. In the rear projection type display apparatus, image light emitted from an image light source such as a CRT (Cathode Ray Tube), an LCD (Liquid Crystal Display) element or a DLP (Digital Light Processing) element is reflected and expanded by a projection mirror and projected to a transmission type screen from the rear side so that a viewer observes a projected image from the front face side of the transmission type screen as well known in the art.
In the rear projection type display apparatus, the incident angle of light from the projection mirror to the transmission type screen is generally set steep in order to reduce the size of the optical system, that is, to reduce the length of the light path. To this end, a Fresnel lens sheet is provided on the transmission type screen for converting the image light from the projection mirror into parallel light, or in other words, for converting the image light into light directed perpendicularly to the plane of the screen.
Fresnel lens sheets are divided into those of the refraction type and those of the reflection type. A Fresnel lens sheet of the refraction type is shown in FIG. 10. Referring to FIG. 10, the Fresnel lens sheet includes a substrate 50 having an emergence face on which prisms (microprisms) 51 having a triangular cross section are arrayed. The Fresnel lens sheet changes the advancing direction of incident light L1 by refracting the incident light L1 at a refracting face 51a of the prisms 51.
A Fresnel lens sheet of the reflection type is shown in FIG. 11. Referring to FIG. 11, the Fresnel lens sheet includes a substrate 50 having an entrance face on which prisms (microprisms) 52 having a triangular cross section are arrayed. The Fresnel lens sheet refracts incident light L1 at refracting faces 52a of the prisms 52 and reflects the refracted light toward the emergence face 50a side of the Fresnel lens sheet at reflecting faces 52b of the prisms 52 to change the advancing direction of the light. The reflected light from the reflecting faces 52b passes through third faces 52c of the prisms 52, which extend in parallel to the emergence face 50a, and emerges from the emergence face 50a of the substrate 50.
Incidentally, one of causes of deterioration of the picture quality of a rear projection type display apparatus is appearance of stray light in the Fresnel lens sheet. This is a phenomenon that part of image light is reflected at a location at which the image light should not be reflected originally and is then repetitively reflected or refracted in or by the Fresnel lens sheet. If the stray light emerges toward the viewer side, then the image looks doubly or triply.
In related art, various techniques have been proposed for suppressing appearance of stray light by a Fresnel lens sheet of the reflection type. One of such techniques is disclosed, for example, in Japanese Patent Laid-Open No. 2004-212770 (refer to paragraphs 0031 to 0045 and FIGS. 1 to 8) (hereinafter referred to as Patent Document 1) wherein a valley portion of each prism is formed in a characteristic shape so that light refracted by a refracting face of the prism or reflected by a reflecting face of the prism and coming to the valley portion may not less likely to make stray light.